Measuring a Wave

A wave repeats the same shape over and over as it travels. Amplitude tells you how tall it is, wavelength how long one repeat is, and frequency how many repeats pass each second.

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In the picture

The glowing curve — one full repeat of it is a cycle.
The dashed centre line — the rest position the wave swings around.
The vertical arrow — the amplitude, from the rest line up to the crest.
The horizontal arrow — the wavelength, from one crest to the next.
The stopwatch — frequency counts cycles per second, measured in hertz.

Examples

A guitar string: pluck it harder for a bigger amplitude (louder), press a fret for a higher frequency (higher pitch).
Ripples on a pond: the crest-to-crest spacing is the wavelength, and how fast a floating leaf bobs is the frequency.

Key takeaways

Amplitude is measured from the rest line to a crest, not from crest to trough.
Speed = frequency × wavelength, so at a fixed speed a higher frequency means a shorter wavelength.